1. Field of the Invention
The present invention relates an apparatus for controlling an automatic transmission. In particular, the present invention relates an apparatus for controlling an automatic transmission which detects failures in solenoid valves disposed in a hydraulic pressure control circuit and carries out a fail-safe control so as to reduce shocks which are generated by the failures.
2. Description of the Related Art
An automatic transmission of a motor vehicle is provided with a torque converter and speed change gears. The automatic transmission automatically provides a gear stage according to a driving condition such as an engine load and a vehicle speed by switching a power-transmitting path of the speed change gears. The switching of the power-transmitting path is carried out by selectively engaging a plurality of frictional elements such as clutches and brakes. The automatic transmission is further provided with a hydraulic pressure control circuit which controls a supply of an operating hydraulic pressure to the respective frictional elements so as to engage or disengage the frictional elements.
Namely, the hydraulic pressure control circuit controls the hydraulic pressure supplied to the respective frictional elements such that a control of gear stages or a speed-change control can be carried out. The hydraulic pressure control circuit is provided with various solenoid valves which generate, supply, discharge and control the operating hydraulic pressure. The solenoid valves controls the operating hydraulic pressure by electrical control signals.
When the solenoid valve fails, a corresponding frictional element is not engaged nor disengaged by a speed-change command output based on the engine load. As a result, a necessary gear stage can not be obtained or the speed change gears are fixed to a neutral condition.
Conventionally, electrical failures such as a braking of wire and a short circuit of respective solenoid valves have been detected in advance by outputting a failure-detecting signal to the respective solenoid valves when staring a motor vehicle. However, even when such electrical failures of the solenoid valves do not occur, the solenoid valves might not work normally by mechanical failures such as a seal failure which is generated by a sticking of plunger ranger or a catching of extraneous materials. In this case, although the electrical failure is not detected, a necessary gear stage can not be obtained or the speed change gears are fixed to a neutral condition.
In order to solve this problem, a fail-safe control has been proposed. Namely, an actual gear stage is detected based on a speed-change commend which is output according to a driving condition, and then a mechanical failure of the solenoid valve is determined based on the detection result and the fail-safe control is finally carried out.
An automatic transmission is generally provided with a frictional element which includes a single hydraulic pressure chamber. Such frictional element is engaged when operating hydraulic pressure is supplied into the chamber, and is disengaged when the operating hydraulic pressure is discharged from the chamber. Another type of the frictional element of an automatic transmission is, for example, disclosed in Japanese Patent Laid-Open Publication No. 8-326888. A 2-4 brake in the Patent Publication includes two hydraulic chambers composed of an engagement chamber and a disengagement chamber. The 2-4 brake is engaged only when operating hydraulic pressure is supplied into the engagement chamber and is disengaged on other conditions. Namely, the 2-4 brake is disengaged when the operating hydraulic pressure is not sullied both the engagement and disengagement chambers, the operating hydraulic pressure is supplied only into the disengagement chamber, or the operating hydraulic pressure is supplied into neither of the engagement chamber nor the disengagement chamber. Further, solenoid valves are respectively provided in a hydraulic pressure control circuit so as to control supply and discharge of the operating hydraulic to and from the engagement chamber and the disengagement chamber so that such frictional element with two hydraulic chamber is controlled.
There is a need to reduce total number of the solenoid valves, simplify a structure of a hydraulic pressure control circuit and simplify software for a control. Therefore, it has been proposed that the engagement chamber and the disengagement chamber of the frictional element are respectively communicated with a single chamber of another frictional element so as to use the solenoid valve in common.
For example, there is provided a first frictional element with two hydraulic pressure chambers composed of an engagement chamber and a disengagement chamber, and a second frictional element with a single hydraulic pressure chamber which is communicated with the disengagement chamber of the first frictional element. In this case, a first gear stage or a low-speed gear stage is established by the first frictional element being engaged and the second frictional element being disengaged. At this time, a first solenoid valve for the engagement chamber supplies operating hydraulic pressure into the engagement chamber of the first element, and a second solenoid valve for the disengagement chamber does not supply the operating hydraulic pressure into the disengagement chamber of the first element and the single chamber of the second element. On the other hand, a second gear stage or a high-speed gear stage is established by the first frictional element being disengaged and the second frictional element being engaged. At this time, the solenoid valve for the engagement chamber supplies the operating hydraulic pressure into the engagement chamber of the first element, and the solenoid valve for the disengagement chamber supplies the operating hydraulic pressure into the disengagement chamber of the first element and the single chamber of the second element.
When a shift-up operation is carried out from the first low-speed gear stage to the second high-speed gear stage, the solenoid valve for the disengagement chamber needs to supply the operating hydraulic pressure into both the disengagement chamber of the first frictional element and the hydraulic chamber of the second frictional element. However, in this operation, a problem might occur. Namely, because of the balance between a mechanical biasing force to the disengagement side of the first frictional element and that of the second frictional element, the first frictional element does not become disengaged although the second frictional element becomes engaged. In this case, the solenoid valve for the disengagement chamber supplies the operating hydraulic pressure into both the disengagement chamber of the first element and the single chamber of the second element. The solenoid valve for the engagement chamber once reduces the operating hydraulic pressure in the engagement chamber of the first element, and then increases the operating hydraulic pressure in the engagement chamber of the first element at a timing when the second element starts to be engaged by a pressure control of the operating hydraulic pressure supplied into the hydraulic chamber of the second element. At the same time, the solenoid valve for the engagement valve increases the operating hydraulic pressures both in the disengagement chamber of the first element and in the hydraulic chamber of the second element to their maximum values. Finally, the first frictional element is fully disengaged and the second frictional element is fully disengaged.
However, for example, suppose the case that a mechanical failure such as a sticking of the solenoid valve for the engagement chamber is detected when the operating hydraulic pressure has been supplied into the engagement chamber of the first element. In this case, the solenoid valve for the engagement chamber can not reduce the operating hydraulic pressure in the engagement chamber of the first element so as to avoid the problem. As explained above, the problem occurs in the shift-up operation from the first low-speed gear stage to the second high-speed gear stage. Thus, the problem still remains, and the first frictional element does not start being disengaged although the second frictional element has already started being engaged. As a result, the second frictional element has low durability and speed-change ability becomes low.